Back in August, Cape Town big-wave surfer Odd Persson was having a bad day. Sunset Reef was hitting 20 ft but strong onshores the day before meant conditions were ‘woolly’ to say the least. Odd had just been caught by a rogue set and was floating in the impact zone, unable to move his arms or legs.
Dougal Patterson and Matt Bromley helped to get Odd to shore. He was taken straight to hospital and found to have spinal-cord shock, concussion and a burst eardrum.
In the process of getting Odd to safety, Dougal let his own board go. The board quickly disappeared in the rip and everybody forgot about it until they knew that Odd was safe and sound. Three weeks later, it turned up at a spot more than 400 kilometres away. I asked Dougal what happened.
“When Matt and I reached Odd we could see straight away that something was wrong. We quickly noticed that he was unable to move – which probably meant a spinal injury.
“So I ditched both his and my boards in order to swim him in without risking damaging his spine any further. The ten-foot boards were smashing against us in the whitewater and so it was too dangerous to drag them behind us. Matt looped his leash under my arm and dragged us behind him by paddling his board, while I lay under Odd supporting his neck and kicking with my legs.
“The rescue services met us on the beach and Odd was rushed to hospital. Miraculously he didn’t have any permanent damage and should be back in the water in a few months. As far as the boards were concerned, I was hoping they would turn up in the next few days. At least mine had my name and phone number written on the bottom.
Apparently the board is in good shape with just a few grazes from the rocks
“Three weeks later I got a call from two non-surfing Afrikaans guys. They had been on a hiking trail in a remote area of the west coast near a spot called Hondeklip Bay, about 425 km from Sunset Reef.
“They spotted my board on the rocks, went down and picked it up and then carried it between them for several hours until they got cellphone reception. Apparently the board is in good shape with just a few grazes from the rocks.”
The fact that the board ended up so far away in such a relatively short time (it moved at an average speed of 800 metres per hour) seemed worth investigating. Why did it go in that direction, and what were the forces that carried it?
I had a few ideas of my own, including the famous Benguela Current that runs up the west coast of Africa. However, I thought it would be better to ask the professionals. So I contacted Mike Hart-Davis from the Nelson Mandela University. Mike is working on a numerical model which predicts the trajectory of objects floating on the sea surface.
In no time at all, Mike had done a hindcast of the trajectory of Dougal’s board. (A hindcast is a forecast of something that has already happened). He was able to predict to a great degree of accuracy the path of Dougal’s board and the place where it ended up. With a bit of further analysis he was able to pinpoint the most significant forces that were contributing the board’s movement.
The model uses a system of ‘virtual particles’ that can be made to start off at a particular position. Their subsequent positions are then plotted as they move over the ocean surface according to the wind and currents. The most likely trajectory of a real object is the average of the trajectories of the virtual particles.
Mike explains: “My model is designed to be used in several applications, one of which is Search and Rescue of objects, people and crafts in the sea, so this was a great test. I ran it for 21 days where the surfboard was lost and then recovered using surface currents and wind. I programmed 1000 virtual particles to represent the characteristics of a surfboard that was lost at sea.
“The average current during the three weeks runs along the coast from southeast to northwest, while during the same time, the average wind blew from southwest to northeast. This resulted in the virtual particles being pushed towards the coast. If it wasn’t for the wind, the virtual particles would be pushed along the coast, but not necessarily hit land where we expected them.
“Because the object (surfboard) is not submerged, the wind becomes an extremely important factor compared to scenarios where the objects would be fully submerged. This case is very interesting for me as it further illustrated the importance of including both current and wind in a Search and Rescue scenarios.
“What is even more remarkable is the model estimated the position of the surfboard really well without us having exact information about the specific time and exact position where the board was lost and recovered. These factors are extremely important as the ocean and wind conditions change by the minute and could impact the accuracy of our predictions."